Binomial Theorem | Formula, Proof, Binomial Expansion & Solved Examples, Practice Problems

πŸ“’ Binomial Theorem – Class 11 Mathematics πŸ“š

πŸš€ Master the Binomial Theorem for Board & JEE Exams! πŸš€

The Binomial Theorem is a fundamental topic in Class 11 Mathematics, widely used in algebra, combinatorics, and calculus. It helps in expanding expressions of the form (a + b)ⁿ efficiently, making it essential for solving complex problems in CBSE Board Exams, JEE Mains, and Advanced.

Binomial theorem is used to find the expansion of two terms hence it is called the Binomial Theorem. Binomial theorem is a fundamental principle in algebra that describes the algebraic expansion of powers of a binomial. According to this theorem, the expression (a + b)n where a and b are any numbers and n is a non-negative integer. It can be expanded into the sum of terms involving powers of a and b.

It gives an expression to calculate the expansion of algebraic expression (a+b)n. The terms in the expansion of the following expression are exponent terms and the constant term associated with each term is called the coefficient of terms.

Binomial Theorem Statement

The Binomial Theorem provides a systematic way to expand the powers of a binomial expression. Binomial theorem for the expansion of (a+b)n is stated as,

(a + b)n = nC0 anb0 + nC1 an-1 b1 + nC2 an-2 b2 + …. + nCr an-r br + …. + nCn a0bn

where n > 0 and the nCk is the binomial coefficient.

Binomial Expansion Formula

Binomial theorem formula gives the expansion of the algebraic identities in the form of a series. This formula is used to find the expansion up to n terms of the (a+b)n. The binomial expansion of (a + b)n can easily be represented with the summation formula.

Binomial Theorem Formula for the expansion of (a + b)n is,

(a + b)n = βˆ‘nr nCr an-r br

where,

  • n is a positive integer,
  • a, b are real numbers, and 0 < r ≀ n

We can easily find the expansion of the various identities such as (x+y)9, (x+12)12, and others using the Binomial Theorem Formula. We can also find the expansion of (ax + by)n using the Binomial Theorem Formula,

Expansion formula for (ax + by)n is,

(ax + by)n = βˆ‘nr nCr (ax)n-r (by)r

where 0< r ≀ n.

Also using combination formula we know that,

Binomial Expansion Formula

nCr = n! / [r! (n – r)!]

Binomial Theorem Proof

We can easily proof Binomial Theorem Expansion using the concept of Principle of Mathematical Induction

Let’s take x, a, n ∈ N, and then the Binomial Theorem says that,

(x+y)n = nC0 xny0 + nC1 xn-1y1 + nC2 xn-2 y2 + … + nCn-1 x1yn-1 + nCn x0yn

Proof:

For n = 1, we get

(x +y)1 = x +y which is true.

For n = 2,

(x +y)2 = (x + y) (x +y)
β‡’ (x +y)2 = x2 + xy + xy + y2 (using distributive property of multiplication over addition)
β‡’ (x +y)2 = x2 + 2xy + y2

which is also true.

Thus, theorem is true for n = 1 and n = 2.

Let’s take a positive integer k.

(x+y)k = kC0 xky0 + kC1 xk-1y1 + kC2 xk-2 y2 + … + kCk-1 x1yk-1 + kCk x0yk

Now consider the expansion for n = k + 1

(x + y) k+1 = (x + y) (x + y)k

β‡’ (x + y) k+1 = (x + y) (xk + kC1 xk-1y1 + kC2 xk-2 y2 + … + kCr xk-ryr +….+ yk)

β‡’ (x + y) k+1 = xk+1 + (1 + kC1)xky + (kC1 + kC2) xk-1y2 + … + (kCr-1 + kCr) xk-r+1yr + … + (kCk-1 + 1) xyk + yk+1

β‡’ (x + y) k+1 = xk+1 + k+1C1xky + k+1C2 xk-1y2 + … + k+1Cr xk-r+1yr + … + k+1Ck xyk + yk+1    

[As we know, nCr + nCr-1 = n+1Cr]

This result is true for n = k+1.

Thus, by concept of mathematical induction, the result is true for all positive integers β€˜n’. Proved.

Binomial Expansion

Binomial Theorem is used to expand the algebraic identity (x + y)n. Hence it is also called the binomial expansion. The binomial expansion of (x + y)n with the help of the binomial theorem is,

(x+y)n = nC0 xny0 + nC1 xn-1y1 + nC2 xn-2 y2 + … + nCn-1 x1yn-1 + nCn x0yn

Using this expansion we get each term in the expansion of (x+y)n

Some special cases from the binomial theorem can be written as:

  • (x + y)n =Β nC0 xn +Β nC1 xn-1 by+Β nC2 xn-2 y2Β + … +Β nCn-1 x yn-1 +Β nCn xn
  • (x – y)n =Β nC0 xn – nC1 xn-1 by +Β nC2 xn-2 y2 + … + (-1)nΒ nCn xn
  • (1 – x)n =Β nC0 – nC1 x +Β nC2 x2 – …. (-1)n nCn xn

Also, nC0nCn = 1

However, there will be (n + 1) terms in the expansion of (a + b)n.

Properties of Binomial Theorem

There are various properties related to the binomial theorem, some of those properties are as follows:

  • Number of Terms: In the binomial Expansion of (x + y)n using the binomial theorem, there are n+1 Β terms and coefficients.
  • First and last terms in the binomial expansion of (x + y)n are xn and yn, respectively.
  • General Term: In the binomial expansion of (x + y)n the general term can be represented as T(r + 1) and is given by T(r + 1) = nCr Γ— x(n-r) Γ— yr.
  • Pascal’s Triangle: The binomial coefficients in the expansion are arranged in Pascal’s triangle which is the pattern of number where each number is the sum of the two numbers above it.
  • Specific Values: When n is a non-negative integer, the expansion simplifies for specific values of n:
    • (a + b)0 = 1
    • (a + b)1 = a + b
    • (a + b)2 = a2 + 2ab + b2
    • (a + b)3 = a3 + 3a2b + 3ab2 + b3
    • and so on…
  • Binomial Coefficients Symmetry: In the binomial expansion of (x + y)n, the rth term from the end is the (n – r + 2)th term from the beginning.
  • If n is even, then in the expansion of (x + y)n, the middle term is ((n/2) + 1). If n is odd, then the middle terms are ((n + 1)/2) and ((n + 3)/2) in the expansion of (x + y)n.

Binomial Expansion for Negative Exponent

Binomial theorem is also used for finding the expansion of the identities which have negative exponents. The coefficients terms in the negative expansion are similar in magnitude to the terms in the corresponding positive exponent.

Some of the simplified expansions of the negative exponents which are widely used are,

  • (1 + x)-1 = 1 – x + x2 – x3 + x4 – x5 + …….
  • (1 – x)-1 = 1 + x + x2 + x3 + x4 + x5 + …….
  • (1 + x)-2 = 1 – 2x + 3x2 – 4x3 + ……..
  • (1 – x)-2 = 1 + 2x + 3x2 + 4x3 + ……..

These expansions are easily proved using binomial expansion and replacing (+) with (-)

Applications of Binomial Theorem

The Binomial Theorem has various applications it is used for a variety of the purposes such as,

  • Finding Remainder in the division of very large numbers.
  • Finding Last Digits of a Number
  • Checking the Divisibility

Finding Remainder using Binomial Theorem

This can easily be understood with the help of the following example.

Example.1: Find the remainder when 297 is divided by 15

Solution:

(297 / 15) = [2(24)24 / 15)] 

= [2(15 + 1)24 / 15]

As each term of the expansion of (1 + 15)24 contains 15 except the first term which is only 1.

Thus, Remainder when 297 is divided by 15 is 1.

Example.2:Β Find the remainder when 7103Β is divided by 25.

Solution:

(7103 / 25) = [7(49)51 / 25)] = [7(50 βˆ’ 1)51 / 25]

= [7(25K βˆ’ 1) / 25] = [(175K – 25 + 25βˆ’7) / 25]

= [(25(7K βˆ’ 1) + 18) / 25]

∴ The remainder = 18

Example.3:Β Β If the fractional part of the number (2403Β / 15) is (K/15), then find K.

Solution:

(2403 / 15) = [2(24)100 / 15]

= 8/15 (15 + 1)100 = 8/ 15 (15Ξ» + 1) = 8Ξ» + 8/15

∡ 8λ is an integer, fractional part = 8/15

So, K = 8.

Finding the Last Digits of a Number

How to find the last digit of an expansion can be understood using the example,

Example.1: Find the last digit of (7)10

Solution:

(7)10 = (49)5 = (50-1)5

(50-1)5 = 5C0 (50)5 βˆ’ 5C1 (50)4 + 5C2 (50)3 βˆ’ 5C3 (50)2 + 5C4 (50) βˆ’ 5C5

            = 5C0 (50)5 βˆ’ 5C1 (50)4 + 5C2 (50)3 βˆ’ 5C3 (50)2 + 5(50) βˆ’ 1

            = 5C0 (50)5 βˆ’ 5C1 (50)4 + 5C2 (50)3 βˆ’ 5C3 (50)2 +  249

A multiple of 50 + 249 = 50K + 249

Thus, the last digit is 9.

Example.2 : Find the last two digits of the number (13)10

Solution :

(13)10 = (169)= (170 βˆ’ 1)5

= 5C0 (170)βˆ’ 5C(170)+ 5C(170)βˆ’ 5C(170)+ 5C(170) βˆ’ 5C5

= 5C(170)βˆ’ 5C(170)+ 5C(170)βˆ’ 5C(170)+ 5(170) βˆ’ 1

A multiple of 100 + 5(170) – 1 = 100K + 849

∴ The last two digits are 49.

Example.3 : The digit in the units place of the numberΒ 183! + 3183.

Solution:

β‡’ 3183 = (34)45.33

β‡’ unit digit = 7 and 183! ends with 0

∴ The units digit of  183! + 3183 is 7.

Relation between Two Numbers

Example.1 :Β Find the larger of 9950Β + 10050Β and 10150

Solution :

10150 = (100 + 1)50 = 10050 + 50 . 10049 + 25 . 49 . 10048 + …

β‡’ 9950 = (100 βˆ’ 1)50 = 10050 β€“ 50 . 10049 + 25 . 49 . 10048 βˆ’ ….

β‡’ 10150 β€“ 9950 = 2[50 . 10049 + 25(49) (16) 10047 + …]

= 10050 + 50 . 49 . 16 . 10047 + … >10050

∴ 10150 β€“ 9950 > 10050

β‡’ 10150 > 10050 + 9950

Divisibility Test

Example.1 :Β Show that 119Β + 911Β is divisible by 10.

Solution :

11+ 911 = (10 + 1)+ (10 βˆ’ 1)11

= (9C. 10+ 9C. 10+ … 9C9) + (11C. 1011 βˆ’ 11C. 1010 + … βˆ’11C11)

= 9C. 10+ 9C. 10+ … + 9C. 10 + 1 + 1011 βˆ’ 11C. 1010 + … + 11C10 . 10βˆ’1

= 10[9C. 10+ 9C. 10+ … + 9C+ 11C. 1010 βˆ’ 11C. 10+ … + 11C10]

= 10K, which is divisible by 10.

Binomial Theorem for Any Index

Binomial Theorem for any index, including non-integer and negative indices, generalizes the familiar binomial expansion that applies to positive integer exponents.

This generalized form involves the use of binomial coefficients that are defined for any real number index using the concept of factorial functions extended to the gamma function for non-integer values.

Multinomial Theorem

We know that the binomial theorem expansion of (x + a)n is,

(x+a)n = nβˆ‘r nCr xn – r ar

Where n∈N

We can generalize this result to get the expansion of,

(x1 + x2 + … +xk)n = βˆ‘(r1 + r2 + …. + rk = n) [n! / r1!r2!…rk!] x1r1 x2r2 …xkrk

The general term in the above expansion is

 [n! / r1!r2!…rk!] x1r1 x2r2 …xkrk

The number of terms in the above expansion is equal to the number of non-negative integral solutions of the equation r1 + r2 + … + rk = n

Each solution of this equation gives a term in the above multinomial expansion. The total number of solutions can be given by n + k – 1Ck βˆ’1.

Binomial Distribution and Binomial Coefficients

Binomial Distribution

The binomial distribution models the number of successes in a fixed number of independent and identical Bernoulli trials (experiments with two possible outcomes: success or failure). It’s characterized by two parameters: nn (the number of trials) and pp (the probability of success in each trial).

Hypergeometric Distribution

The hypergeometric distribution models the probability of obtaining a specific number of successes in a sample of a fixed size drawn without replacement from a finite population containing a known number of successes and failures.

Important Formulae:

  • The number of terms in the expansion of (x1Β + x2Β + … xr)nΒ is (n + r βˆ’ 1)Cr – 1
  • The sum of the coefficients of (ax + by)nΒ is (a + b)n

If f(x) = (a+ a1x + a2x+ …. + amxm)n then

  • (a) Sum of coefficients = f(1)
  • (b) Sum of coefficients of even powers of x is: [f(1) + f(βˆ’1)] / 2
  • (c) Sum of coefficients of odd powers of x is [f(1) βˆ’ f(βˆ’1)] / 2

Binomial Theorem for Rational Index

The number of rational terms in the expression of (a1/l + b1/k )n is [n / LCM of {l,k}] when none of and is a factor of and when at least one of and is a factor of is [n / LCM of {l,k}] + 1 where [.] is the greatest integer function.

Example.1 : Find the number of irrational terms in (8√5 + 6√2)100.

Solution :

Tr + 1 = 100Cr (8√5)100 – r . (6√2)= 100C. 5[(100 – r)/8] .2r/6.

∴ r = 12,36,60,84

The number of rational terms = 4

The number of irrational terms = 101 – 4 = 97

Number of Terms in the Expansion

Number of Terms in the Expansion ofΒ (x1Β + x2Β + … + xr)n

From the general term of the above expansion, we can conclude that the number of terms is equal to the number of ways different powers can be distributed to x1, x2, x…., xn, such that the sum of powers is always β€œn”.

The number of non-negative integral solutions of

x1Β + x2Β + … + xrΒ = n isΒ n +r – 1Cr – 1.

For example, the number of terms in the expansion of

(x + y + z)3Β isΒ 3 + 3 -1C3 – 1Β =Β Β 5C2Β = 10

As in the expansion, we have terms such as

As xy0 z0, x0 y1 z2, x0 y2 z1, x0 y3 z0, x1 y0 z2, x1 y1 z1, x1 y2 z0, x2 y0 z1, x2 y1 z0, x3 y0 z0.

The number of terms in (x + y + z)n is n + 3 – 1C3 – 1 n + 2C2.

The number of terms in (x + y + z + w)n is n + 4 – 1C4 – 1 = n + 3C3 and so on.

Important Sum of Binomial Coefficients

Some properties of combination which are widely used in simplifying binomial expansion are,

  • C1 + C2 + C3 + C4 + …….Cn = 2n
  • C0 + C2 + C4 + …. = C1 + C3 + C5 + ……. = 2n-1
  • C0 – C1 + C2 – C3 + C4 – C5 + …. = 0
  • C1 + 2C2 + 3C3 + 4C4 + …….nCn = n2n-1
  • C1 – 2C2 + 3C3 – 4C4 + …….(-1)nnCn = 0
  • C12 + C22 + C32 + C42 + …….Cn2 = (2n)! / (n!)2

Pascal’s Triangle Binomial Expansion

The number associated with the terms of the binomial expansion is called the coefficient of the binomial expansion. These variables can easily be found using Pascal’s Triangle or by using combination formulas.

Binomial Theorem Coefficients

After examining the coefficient of the various terms in the expansion of algebraic identities using the binomial theorem, we have observed a trend in the coefficient of the terms of the expansion. The trend is that the coefficient of the terms in the expansion of the binomial terms is directly similar to the rows of the Pascal triangle.

For example, the coefficient of each term in the expansion of the (x+y)4 is directly equivalent to the terms in the 4th row of the Pascal Triangle.

Pascal Triangle

Pascal Triangle is an alternative method of the calculation of coefficients that come in binomial expansions, using a diagram rather than algebraic methods.

In the diagram shown below, it is noticed that each number in the triangle is the sum of the two directly above it. This pattern continues indefinitely to obtain coefficients of any index of the binomial expression.

When we observe the pattern of the coefficients of the expansion (a + b)n, Pascal’s triangle for the pattern of the coefficients of the expansion (a + b)7 is shown in the figure below:

Pascal Triangle

Thus, from the above diagram, the expansion of small powers of n (e.g. n=0, 1, 2, 3, 4, 5, 6, 7) can be calculated as:

  • (a + b)0 = 1
  • (a + b)1 = a + b
  • (a + b)2 = a2 + 2ab +b2
  • (a + b)3 = a3 + 3a2b + 3ab2 + b3
  • (a + b)4 = a4 + 4a3b + 6a2b2 + 4ab3 + b4
  • (a + b)5 = a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + b5
  • (a + b)6 = a6 + 6a5b + 15a4b2 + 20a3b3 + 15a2b4 + 6ab5 + b6
  • (a + b)7 = a7 + 7a6b + 21a5b2 + 35a4b3 + 35a3b4 + 21a2b5 + 7ab6 + b7

In this way, the expansion from (a + b)0 to (a + b)7 is obtained by the application of Pascal’s Triangle. But finding (a + b)15 is really a long process using Pascal’s triangle. So, here we use combinations formula.

Combinations

The combination formulas are also widely used to find coefficients of the various terms in the expansion of the binomial theorem.

The combination formula used for choosing r objects out of n total objects is widely used in the binomial expansion and it is defined as,

nCr = n! / [r! (n – r)!]

Also, some common combination formulas used in the Binomial Expansion are,

  • nCn = nC0 = 1
  • nC1 = nCn-1 = n
  • nCr = nCr-1

Solved Examples of Binomial Theorem

Example.1: Find the value of (x+y)2 and (x+y)3 using Binomial expansion.

Solution:

(x+y)2 = 2C0 x2y0 + 2C1 x2-1y1 + 2C2 x2-2 y2

β‡’ (x+y)2 = x2 + 2xy + y2

And (x+y)3 = 3C0 x3y0 + 3C1 x3-1y1 + 3C2 x3-2 y2 + 3C3 x3-3 y3

β‡’ (x+y)3 = x3 + 3x2y + 3xy2 + y3

β‡’ (x+y)3 = x3 + 3xy(x+y) + y3

Example.2 : Find the expansion of (x + 5)6 using the binomial theorem.

Solution:

We know that here,

(a + b)n = nC0 anb0 + nC1 an-1 b1 + nC2 an-2 b2 + …. + nCr an-r br + …. + nCn a0bn

Thus, (x + 5)6 = 6C0 x650 + 6C1 x6-151 + 6C2 x6-252 + 6C3 x6-353 + 6C4 x6-454 + 6C5 x6-555 + 6C6 x6-656

 = 6C0 x6 + 6C1 x55 + 6C2 x452 + 6C3 x353 + 6C4 x254 + 6C5 x155 + 6C6 x056

 = x6 + 30x5 + 375x4 + 2500x3 + 9375x2 + 18750x + 15625

Example 3: Expand the binomial expression (2x + 3y)2.

Solution:

(2x + 3y)2

= (2x)2 + 2(2x)(3y) + (3y)2

= 4x2 + 12xy + 9y2

Example 4: Expand the following (1 – x + x2)4

Solution:

Put 1 – x = y

Then,

(1 – x + x2)4 = (y + x2)4

= 4C0y4(x2)0 + 4C1y3(x2)1 + 4C2y2(x2)2 +4C3y(x2)3 +4C4(x2)4

= y4 + 4y3x2 + 6y2x4 + 4yx6 + x8

= (1 – x)4 + 4(1 – x)3x2 + 6(1 – x)2x4 + 4(1 – x)x6 + x8

= 1 – 4x + 10x2 – 16x3 + 19x4 – 16x5 + 10x6– 4x7 + x8

Example 5: Find the binomial expansion of (x3 + 1)3.

Solution:

We know that,

(x + y)n = nC0 xny0 + nC1 xn-1y1 + nC2 xn-2 y2 + … + nCn-1 x1yn-1 + nCn x0yn

(x3 + 1)3 = 3C0 (x3)310 + 3C1  (x3)211 + 3C2 (x3)112+ 3C3 (x3)013

(x3 + 1)3 = (1)x9 + (3)x6 + (3)x3 + (1)

(x3 + 1)3 = x9 + 3x6 + 3x3 + 1

Example.6: Expand: [x2 + (3/x)]4, x β‰  0

Solution:

[x2 + (3/x)]4

Using binomial theorem,

[x2 + (3/x)]44C0 (x2)44C1 (x2)3 (3/x) + 4C2 (x2)2 (3/x)24C3 (x2) (3/x)34C4 (3/x)4

= x8 + 4 x6 (3/x) + 6 x4 (9/x2) + 4 x2 (27/x3) + (81/x4)

= x8 + 12x5 + 54x2 + (108/x) + (81/x4)

Example 7: Compute (98)5

Solution:

Let us write the number 98 as the difference between the two numbers.

98 = 100 – 2

So, (98)5 = (100 – 2)5

Using binomial expansion,

(98)55C0 (100)5 – 5C1 (100)4 (2) + 5C2 (100)3 (2)2 – 5C3 (100)2 (2)35C4 (100) (2)4 – 5C5 (2)5

=  10000000000 – 5 Γ— 100000000 Γ— 2 + 10 Γ— 1000000 Γ— 4 – 10 Γ—10000 Γ— 8 + 5 Γ— 100 Γ— 16 – 32

= 10040008000 – 1000800032 

= 9039207968

Example.8 : Β Find the number of terms in (1 + 2x +x2)50

Solution :

(1 + 2x + x2)50 = [(1 + x)2]50 = (1 + x)100

The number of terms = (100 + 1) = 101

Binomial Theorem – FAQs

What is Binomial Theorem?

Binomial theorem is the basic theorem which provides an expansion to algebraic identity such as (x+y)n

(x+y)n = nC0 xny0 + nC1 xn-1y1 + nC2 xn-2 y2 + … + nCn-1 x1yn-1 + nCn x0yn

What is General Term in Binomial Theorem?

General term is the binomial theorem expansion of (x+y)n is,

 Tr+1 = nCr xn-ryr

What Is Constant Term in the Binomial Theorem?

Term in the binomial theorem expansion which is free from the variables is called the constant term.

What is the Binomial Theorem formula?

Binomial theorem is used to find the expansion of the algebraic terms of the form(x + y)n. The formula of the binomial expansion is,

(x + y)n = Ξ£rnnCr xn – r Β· yr

What are the number of terms in the expansion of (x + a)n + (x-a)n?

The number of terms in the expansion of (x + a)n + (x-a)n is given with the help of the formula,

  • If n is even (n+2)/2Β 
  • If n is odd (n+1)/2

πŸ” Applications of the Binomial Theorem

πŸ“Œ Algebra & Number Theory – Polynomial expansion, finding coefficients
πŸ“Œ Probability & Statistics – Combinations & binomial distributions
πŸ“Œ Physics & Engineering – Approximation in equations of motion
πŸ“Œ Computer Science & Coding – Algorithm efficiency & complexity analysis

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βœ”οΈ Concept-Based Learning – Step-by-step proofs & examples
βœ”οΈ Exam-Oriented Approach – Covers CBSE Board & JEE syllabus
βœ”οΈ Practice Problems – Includes solved examples & exercises

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